namespace in C++ | Set 2 (Extending namespace and Unnamed namespace)

Difficulty Level : Easy
Last Updated : 29 May, 2017

We have introduced namespaces in below set 1.

It is also possible to create more than one namespaces in the global space. This can be done in two ways.

namespaces having different names

// A C++ program to show more than one namespaces  
// with different names. 
#include <iostream> 
using namespace std; 
  
// first name space 
namespace first 
{ 
   int func() {  return 5; } 
} 
  
// second name space 
namespace second 
{ 
   int func() { return 10; } 
} 
  
int main() 
{ 
   // member function of namespace 
   // accessed using scope resolution operator 
   cout << first::func() <<"\n";         
   cout << second::func() <<"\n";  
   return 0; 
} 

Output:

5
10

Extending namespaces (Using same name twice)
It is also possible to create two namespace blocks having the same name. The second namespace block is nothing but actually the continuation of the first namespace. In simpler words, we can say that both the namespaces are not different but actually the same, which are being defined in parts.

// C++ program to demonstrate namespace exntension 
#include <iostream> 
using namespace std; 
  
// first name space 
namespace first  
{  
   int val1 = 500;   
} 
  
// rest part of the first namespace 
namespace  first  
{  
   int val2 = 501;   
} 
  
int main() 
{ 
   cout << first::val1 <<"\n";         
   cout << first::val2 <<"\n";  
   return 0; 
} 

Output:

500
501

Unnamed Namespaces

They are directly usable in the same program and are used for declaring unique identifiers.
In unnamed namespaces, name of the namespace in not mentioned in the declaration of namespace.
The name of the namespace is uniquely generated by the compiler.
The unnamed namespaces you have created will only be accessible within the file you created it in.
Unnamed namespaces are the replacement for the static declaration of variables.

// C++ program to demonstrate working of unnamed  
// namespaces 
#include <iostream> 
using namespace std; 
  
// unnamed namespace declaration 
namespace 
{ 
   int rel = 300;  
} 
  
int main() 
{ 
   cout << rel << "\n"; // prints 300 
   return 0; 
} 

Output:

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